There is often a need to maintain the focus of a laser beam on a surface. One example lies in the inspection of a photolithographic masks. These masks have a chrome pattern on a glass or quartz substrate and are used in the manufacture of thousands of semi-conductor wafers during a production run in a "stepper" printing machine. Therefore, it is critical that the surface of the mask be free of contaminating particles lest the images of the particles show up on the wafers causing defects. Accordingly, the masks are typically inspected using very precise equipment shown, for example, in U.S. Pat. Nos. 4,943,734; 4,794,264; 4,794,265; and 5,389,794 incorporated herein by reference.
As a mechanical mask holder/spindle assembly spins the mask, the surface of the mask is illuminated by a laser beam directed to the surface and the scattering of the laser beam from the surface is analyzed: the scattering by the surface is different if a flaw or particle is present than if no particle or flaw is present. In fact, the scattering is indicative of the size of a detected particle flow. In order to scan for and detect very small particles (e.g., 0.3 microns in diameter), on a patterned plate, it is very important that the laser beam be focused to form a very small spot size. For a small diffraction-limited spot size, the depth of focus is very small. Therefore, the distance from the means which focuses the laser beam to the surface must be kept constant. Since the mask may be warped and/or since the mechanical mask holder/spindle assembly may cause displacement of the mask with respect to the parabolic mirror, however, the laser beam may become defocused on the mask surface. When it does, it will not be able to detect small particles on the surface of the mask. Accordingly, any displacement of the mask must be detected and the distance from the parabolic mirror to the mask adjusted to keep the laser beam in focus at all times as the mask rotates.
There are a few prior art techniques for detecting deflection of a surface in a direction normal to the surface. The beam deflection technique detects movement of a laser beam after it is deflected by the surface. Movement of the beam after it is reflected from the surface is indicative of surface displacement. The problem with the beam deflection technique, however, is that it is sensitive to the angle of the surface. Therefore, it cannot be used in conjunction with warped photolithographic masks. Another technique is called the "triangulation". See "Optical Techniques for Industrial Inspection", P. Cielo, Academic Press; 1988. Although this technique overcomes the surface angle sensitivity problem of the beam deflection technique, triangulation can not be used on surfaces having areas of differing reflectivity such as a photolithographic mask which has chrome areas (high reflectivity) and glass areas (low reflectivity). A third technique is the knife edge technique also known as the Foucault technique. In this technique a small focused spot is imaged onto a knife edge, and a detector, usually a bicell, is placed behind the knife edge. This technique is also sensitive to a chrome on glass pattern, for example, when a chrome edge crosses the focused light spot. A fourth technique, called image processing, involves the observation of the sharpness or focus of the chrome/glass pattern image on a detector. If the image of the pattern goes out of focus, the surface must have moved with respect to the detector. The problem with the image processing technique, however, is that it depends on a pattern always being in view of the image processor. This may not always be possible since some photolithographic masks are not patterned in all areas of the mask.